At the interface between the surface of a solid object and air, the convective boundary condition must be enforced:
<math>-k \frac{\partial T}{\partial n} = -h \left[ T(\mathbf{r}) - T_{\infinfin} \right] </math>
where T<sub>∞</sub> is the ambient temperature, and h is the coefficient of convective heat transfer having units of W/(m<sup>2</sup>.K).